In a turbocharger mounted to a vehicle, a ship, and an industrial engine, used is a compressor which includes an impeller that rotates at a high speed and increases a pressure of a fluid taking advantageous of a centrifugal force. For such a compressor of a turbocharger, it is desirable to achieve a wide flow-rate operation range in terms of the performance of an engine, for example, to improve the engine torque performance or to increase the engine output.
As a measure to expand the flow-rate operation range of such a compressor, there is a pre-swirl generation device in which a variable mechanism such as guide vanes is disposed at the upstream side of the compressor, and the variable mechanism is controlled to generate a swirl flow in the fluid, so as to expand the flow-rate operation range (Patent Document 1).
Further, an application previously filed by the present applicant and not published yet (Patent Document 2) discloses a technique to expand the flow-rate operation range by modifying the shape of an intake pipe at the upstream side of a compressor and generating a swirl flow in a fluid flowing through the intake pipe.